In recent years, regarding an image forming apparatus for forming an image on recording paper, the following structure has been proposed. With this structure, the recording paper adheres to and is held on a transfer drum which is in contact with and rotates together with a photoreceptor drum. An electrostatic latent image formed on the photoreceptor drum is developed into a toner image by attracting toner to the electrostatic image, and the toner image is then transferred to transfer paper wound around the transfer drum.
For example, as illustrated in FIG. 31, an image forming apparatus of this type includes a cylinder 101 provided with a dielectric layer 101a as the transfer drum. Disposed separately inside the cylinder 101 are a corona charger 102 for attracting transfer paper P, and a corona charger 104 for transferring a toner image formed on a surface of a photoreceptor drum 103 to the transfer paper P. The adhesion and transfer of the transfer paper P are separately carried out by the charger 102 and 104.
An image forming apparatus shown in FIG. 32 includes a cylinder 201 having a double-layer structure formed by a semi-conductive layer 201a as an outer layer and a substrate 201b as an inner layer, and a grip mechanism 202 for holding the transported transfer paper P around the cylinder 201. Then, the toner image on the photoreceptor drum 103 is transferred to the transfer paper P by applying a voltage to the semi-conductive layer 201a as the outer layer of the cylinder 201 or charging a surface of the cylinder 201 by discharges of a charger in the cylinder 201.
However, in the image forming apparatus shown in FIG. 31, since the cylinder 101 as the transfer roller has a single-layer structure formed by only the dielectric layer 101a, it is necessary to dispose the corona chargers 102 and 104 therein. This structure restricts the size of the cylinder 101, and prevents a reduction in the size of the image forming apparatus.
Whereas in the image forming apparatus shown in FIG. 32, since the cylinder 201 as the transfer roller has a double-layer structure, it is possible to reduce the number of chargers for charging the cylinder 201 so as to transfer the toner image to the transfer paper P. However, since the grip mechanism 202 is included in the image forming apparatus, the overall structure of the apparatus becomes complicated. As a result, the total number of component parts in the apparatus and the manufacturing cost of the apparatus are increased.
In order to solve the above problems, for example, Japanese Publication for Unexamined Patent Application No. 74975/1990 discloses a structure in which a transfer drum is formed by laminating a grounded metal roller with a conductive rubber and a dielectric film, and a corona charger is disposed in the vicinity of a position where transfer paper is separated from the transfer drum. In this structure, the corona charger is driven by a unipolar power source.
In this image forming apparatus, the transfer paper is attracted to the transfer drum by inducing charges on a dielectric film by means of the corona charger. When the transfer paper adheres to the transfer drum, more charges are induced, thereby allowing a transfer of an image.
In the image forming apparatus, a surface of the transfer drum is charged by a single charger so as to attract transfer paper and transfer the image to the transfer paper. Since only one charger is necessary, a reduction in the size of the transfer drum is achieved. Moreover, since a mechanism such as a grip mechanism is not required to hold the transfer paper, it is possible to attract the transfer paper with a simplified structure.
However, in the image forming apparatus disclosed in the above-mentioned publication, the surface of the transfer drum is charged by atmospheric discharges of the corona charger. Therefore, when forming a color image, i.e., when executing a transfer process a plurality of times, charges are supplied by the corona charger every time a transfer is completed. It is thus necessary to include a charger unit formed by, for example, a unipolar power source. This causes increases in the number of component parts of the apparatus and the manufacturing cost of the apparatus.
When the surface of the transfer drum is scratched and when charging is carried out by atmospheric discharges, an electric field becomes smaller and loses its balance at the scratched area.. Consequently, a transfer defect occurs, for example, a blank portion is produced at the scratched area, lowering the image quality.
Additionally, since the surface of the transfer roller is charged by the atmospheric discharges, an increased voltage is required for charging, and the driving energy of the image forming apparatus becomes larger. Furthermore, since the atmospheric discharges are easily affected by environmental conditions such as the temperature and moisture in the air, the surface potential of the transfer roller tends to be varied. As a result, failure in attracting the transfer paper and disorderly images are likely to occur.
Recently, a copying machine capable of printing a copy of an image on a sheet which is commonly used for clerical work, a plastic sheet, a post card, an envelope or a label (hereinafter just referred to as the transfer paper) as well as on a special copy sheet (so-called copy paper or ordinary paper) is known. As illustrated in FIG. 33, a transfer device (see Japanese Publication for Examined Patent Application No. 25235/1967, for example) for use in such a copying machine includes a transfer drum 90 for transferring a toner image formed on a surface of a photoreceptor drum (not shown) to transfer paper 99. Disposed in an upstream section of the transfer drum 90 in a transfer-paper transport path 95 are transport rollers 91 and register rollers 92. A pair of curl rollers formed by a hard roller 93 and a soft roller 94 are provided between the transfer drum 90 and register rollers 92. The hard roller 93 curls up the transfer paper 99 in an arc along the transfer drum 90 so that the transfer paper 99 is easily wound around the transfer drum 90.
The soft roller 94 is pressed against the hard roller 93 so that the hard roller 93 cuts into the soft roller 94 at the contact position. Therefore, when the transfer paper 99 passes through the contact position between the hard roller 93 and the soft roller 94, the transfer paper 99 curls up toward the hard roller 93. The transfer paper 99 is thus easily wound around the transfer drum 90. Here, the transfer paper 99 is wound and held on the transfer drum 90, for example, by electrostatic adhesion.
However, in the conventional copying machine, the hard roller 93 and the soft roller 94 are positioned in a downstream section of the soft roller 94 and the register rollers 92 on the transfer-paper transport path 95. Therefore, even if the transfer paper 99 is supplied by the register rollers 92 to the transfer drum 90 at a predetermined time in relation with the image formation, the timing may vary at the time the transfer paper 99 passes through the contact position between the hard roller 93 and the soft roller 94. If the transfer paper 99 is not supplied to the transfer drum 90 at the predetermined time, the toner image is transferred to a displaced position on the transfer paper 99, resulting in an undesired transfer. Moreover, when the hard roller 93 and the soft roller 94 are provided, the transfer-paper transport path 95 becomes longer. Consequently, it becomes harder to supply the transfer paper 99 to the transfer drum 90 at the predetermined time, and the possibility of a so-called paper jam is increased.
In addition, since the hard roller 93 and the soft roller 94 are fixed in predetermined positions, the amount of cut of the hard roller 93 to the soft roller 94 is always uniform. Specifically, the degree of curl of the transfer paper 99 can not be changed depending on the type, paper quality and thickness of the transfer paper 99. Namely, since there are variations in the degree of curl of the transfer paper 99 depending on the type, paper quality and thickness of the transfer paper 99, it is difficult to stably supply the transfer paper 99 having a uniform degree of curl to the transfer drum 90.
For instance, when the transfer paper 99 is thin and soft, the degree of curl of the transfer paper 99 becomes too large and the transfer paper 99 is tightly wound around the transfer drum 90. It is therefore hard to separate the transfer paper 99 from the transfer drum 90, and paper jam is apt to occur. On the other hand, if the transfer paper 99 is thick and hard, the degree of curl of the transfer paper 99 becomes too small. As a result, the transfer paper 99 is hard to be wound around the transfer drum 90, and the toner image tends to be transferred in a displaced position on the transfer paper 99.
Moreover, if an envelope is used as the transfer paper 99, the envelope is likely to be crinkled because the degree of curl can not be adjusted. If a label formed by an adhesive label paper and back paper is used as the transfer paper 99, when the label paper is curled up, it tends to separate from the back paper. When the degree of curl is set to a value optimum for the envelope or the label, if transfer paper other than the envelope and the label is used as the transfer paper 99, the degree of curl becomes too small. Therefore, the transfer paper 99 is hard to be wound around the transfer drum 90, and the toner image is transferred to a displaced position on the transfer paper 99.
Thus, there is a demand for an image forming apparatus capable of curling transfer paper without changing the timing for supplying the transfer paper to the transfer drum. There is also a demand for an image forming apparatus capable of freely adjusting the degree of curl of transfer paper depending on the type, paper quality and thickness of the transfer paper.
In order to separate the transfer paper to which the toner image has been transferred from the transfer drum, a separating claw with a pointed edge is provided so that it comes into contact with and out of contact with an outer surface of the transfer drum. The separating claw is conventionally arranged in various positions on a transport path of the transfer paper. For example, Japanese Publication for Examined Patent Application 52446/1980 discloses an electrophotographic copying machine including a heat roller 81 and a pressure roller 82 which are in contact with each other and rotate together, and a separating claw 83 having a pointed edge which comes into contact with a surface of the heat roller 81 as shown in FIG. 34. In this case, the toner image on recording paper 84 is fixed when the recording paper 84 passes between the rollers 81 and 82. The separating claw 83 prevents the recording paper 84 from being wound around the heat roller 81.
The following description discusses an example where such a claw is provided for separating transfer paper adhering to the transfer drum. As illustrated in FIG. 35, a transfer drum 86 which attracts and holds transfer paper P thereon is disposed in contact with the photoreceptor drum 85. A separating claw 87 is provided so that it comes into contact and out of contact with an outer surface of the transfer drum 86. A substantially V-shaped space S is formed between the outer surfaces of the photoreceptor drum 85 and transfer drum 86 in a transfer region where the drums 85 and 86 are in contact with each other, located near a fixing unit 88. The installation position of the separating claw 87 is determined so that the edge of the separating claw 87 enters into the space S as far as possible to come into contact with the outer surface of the transfer drum 86 at a position closer to the contact position between the photoreceptor drum 85 and transfer drum 86.
However, in the conventional apparatus, since the transfer paper P is separated from the transfer drum 86 near the contact position between the photoreceptor drum 85 and transfer drum 86 and sent to the fixing unit 88, distortion of an image may occur or paper jam may be caused by unsatisfactory transport of the transfer paper.
Namely, with the above-mentioned configuration, as illustrated in FIG. 36, after the transfer paper P is separated by the separating claw 87, it is guided to an upward direction along the separating claw 87. Moreover, the transfer paper P which has been attracted to and held on the outer surface of the transfer drum 86 is curled according to a curvature of the outer surface of the transfer drum 86. Therefore, when the attracting and holding forces for the transfer paper P are cancelled after the transfer paper has passed through a separating point where the separating claw 87 comes into contact with the outer surface of the transfer drum 86, it is not certain that whether the transfer paper P is moved along the surface of the separating claw 87. In addition, after the transfer paper P passes through the separating point, it is moved in the upward direction while being undulated and deviated. As a result, as illustrated by the broken line in FIG. 36, there is a possibility that the transfer paper is bent nearly at 90.degree. at the separating point. At this time, the bent transfer paper comes into contact with the photoreceptor drum 85 and a cleaning unit 89 which are located on a side toward which the transfer paper is bent. This may cause distortion of an image and paper jam.
Furthermore, in the conventional apparatus, after the front edge of the transfer paper P starts to be separated from the transfer drum, the separating claw 87 is kept in contact with the transfer drum 86 at least until the rear edge of the transfer paper P passes through the separating point. Thereafter, the claw 87 is controlled to be separated from the surface of the transfer drum 86. Since the contact time in which the separating claw 87 is in contact with the transfer drum 86 is long, the surface of the transfer drum 86 tends to be damaged, for example, scratched.